The use of circuit breakers is widespread in modern-day residential, commercial and industrial electric systems, and they constitute an indispensable component of such systems toward providing protection against over-current conditions. Various circuit breaker mechanisms have evolved and have been perfected over time on the basis of application-specific factors such as current capacity, response time, and the type of reset (manual or remote) function desired of the breaker.
One type of circuit breaker mechanism employs a thermo-magnetic tripping device to "trip" a latch in response to a specific range of over-current conditions. The tripping action is caused by a significant deflection in a bi-metal or thermostat-metal element which responds to changes in temperature due to resistance heating caused by flow of the circuit's electric current through the element. The thermostat metal element is typically in the form of a blade and operates in conjunction with a latch so that blade deflection releases the latch after a time delay corresponding to a predetermined over-current threshold in order to "break" the current circuit associated therewith.
Another type of circuit interruption arrangement, useful for interrupting circuits having higher current-carrying capacities, uses current transformers to induce a current corresponding to the current in the circuit path, and a rectification circuit to condition this induced current before charging a power supply capacitor, which provides the operating power to the arrangement, and before presentation to an electronic circuit measuring the induced current and detecting faults in the circuit path. In response to a power fault being detected, the electronic circuit generates a control signal to actuate a solenoid (or equivalent device) to cause the circuit-interrupting contacts to separate and interrupt the circuit path.
A primary difficulty in designing such a self-powered circuit interruption arrangement concerns the type of current transformers used to induce the current from the circuit path and the circuit design for conditioning the induced current. The current transformers must be able to deliver the required current for operating the electronic measuring circuit at a suitable power supply voltage, for example, about 12 volts. At the same time, the induced current needs to be as sinusoidal as possible so that an accurate measurement can be made for the purposes of determining whether or not to trip in the presence of an overload or other type of fault. An objective for many current-interruption applications is to meet these requirements using the smallest possible current transformers and with a primary excitation current which is extremely low, for example, of 3 amps or less.
This objective has not been adequately addressed in known circuit interruption arrangements. For example, using a conventional isolated Wye connection, as shown in FIG. 1, each of the current transformers 20 provides induced current to a three-phase rectifier 24 having a positive output 26 and a negative output 28. The positive output 26 is used to deliver the required current for charging a capacitor 30 and operating the measurement electronics circuit 32. A first clamping circuit comprising a 12 volt zener diode 34 and a blocking diode 36 are used to ensure that the capacitor charges positively and at voltages not above 12 volts. At the other side of the rectifier 24, the negative output 28 is used to return power supply current and to provide a current signal from which the measurement electronics circuit 32 can accurately analyze for power faults. A burden resistor 38 is used to convert the current signal to the voltage signal used by the measurement electronics circuit 32 for the analysis. In this arrangement, the current required at the positive output 26 for waking (or powering) up the measurement electronics circuit 32 is significantly less than that required at the negative output 28 for sinusoidal fidelity.
A significant improvement over this conventional isolated wye connection is the approach illustrated and described in U.S. Pat. Nos. 4,041,540 (Kampf et al.) and 4,048,663 (Lemke), and assigned to the instant assignee. See also U.S. Pat. No. 4,121,269. In this approach, a grounded wye connection similar to the arrangement of FIG. 1 is used but modified in that the component interconnection point for the zener diode 34, the resistor 38 and the capacitor 30 is also connected to circuit common (ground) at the current transformers (hereinafter, the "Kampf-modified arrangement"). Thus, two current paths are effectively produced. One current path conducts positive half-cycles from the positive output 26 of the current transformers, through the power supply capacitor 30 and the zener diode 34, while the other current path conducts negative half-cycles through the burden resistor. Each current path returns via the connection to circuit common (ground) at the current transformers.
Since the burden resistor 38 in this modified configuration produces much less voltage drop across the current transformers than does the power supply voltage, the required fidelity of the current in the negative half-cycle is obtained with much lower primary current than before. Since the current for the power supply is now being supplied only during the positive half-cycles, much more primary current is now required to deliver the power supply's needs. Also since the volt-seconds produced must be the same on the positive and negative half-cycles, and since the positive half-cycle must produce much more voltage to conduct than the negative half-cycle, the current transformers will conduct for a much shorter time in the positive direction. Unlike the unmodified operation of the arrangement of FIG. 1, this means that the current required at the positive output 26 for waking (or powering) up the measurement electronics circuit 32 is significantly more than that required at the negative output 28 for sinusoidal fidelity.
Accordingly, the aforementioned requirements are not met by either type of approach, and there is a need for a circuit interruption arrangement which overcomes the aforementioned shortcomings.